In order to manufacture shaped articles from those materials, it is usually necessary to use sintering aids. Owing to their known inertness to sintering, neither silicon nitride powder nor silicon carbide powder can be highly densified under the conditions of conventional hot-pressing or pressure-sintering processes without the concomitant use of additives which promote sintering. High density alone, however, is not the sole criterion for excellent properties especially for the high-temperature strength of the article. High temperature strength as used in the present application means that the flexural strength at room temperature remains virtually unchanged, or shows only a slight decrease, at high temperatures up to about 1400.degree. C.
From a consideration of the properties of silicon nitride and silicon carbide, it was believed that articles formed from a mixture of the materials would have advantageous properties. Articles manufactured from pulverulent silicon nitride together with less than 40% by volume of pulverulent silicon carbide and magnesium oxide as a sintering aid, by simple hot pressing at 1750.degree. C. under a die pressure of 30 MPa (4000 p.s.i.), are known (see U.S. Pat. Nos. 4,184,882 and 4,187,116). The shaped articles manufactured as described above possess flexural strength values at room temperature that are lower than the flexural strength value for silicon nitride alone. However, the articles show a smaller decrease in flexural strength at 1400.degree. C. than articles manufactured from silicon nitride alone. Although the original decrease in flexural strength of 80% shown by silicon nitride alone could be reduced to approximately 50%, it is still very high, and thus mixed materials of that type cannot be considered to be resistant to high temperature insofar as their mechanical strength is concerned.
A further improvement in the properties of articles based on mixtures of silicon nitride and silicon carbide is clearly not possible using the conventional hot pressing methods because the necessary concomitant use of sintering aids promotes the formation of secondary phases at the grain boundaries. Even in low concentrations, the secondary phases can interfere with the high temperature strength of the articles.
A process for the manufacture of shaped articles consisting of silicon nitride alone, which can be carried out without the use of sintering aids is disclosed in DE-OS No. 25 48 740 (which corresponds to GB-PS No. 1 522 705). In that process, silicon nitride powder having not more than 0.6% by weight of impurities in the form of foreign metal oxides, or articles preshaped therefrom, are sealed in glass capsules having a high softening temperature and are isostatically hot-pressed in a high-pressure autoclave using a gas as a pressure-transfer medium, at temperatures of from 1700.degree. to 1800.degree. C. and a pressure of from 150 to 1500 MPa. The shaped articles, of almost theoretical density, which can be obtained in that manner have a flexural strength of approximately 600N/mm.sup.2 at room temperature, which shows a decrease of approximately 45% at 1370.degree. C. (see H. Larker et al. in SAE (Tech. Pap.) 1977, 770335; reference in Chemical Abstracts, Vol. 87 (1977) 171828 b). The problem, therefore, is to make available substantially pore-free shaped articles of mixed materials based on silicon nitride and silicon carbide which can be manufactured by densifying a mixture of Si.sub.3 N.sub.4 powder and pure SiC powder in any desired quantitative proportions without the concomitant use of sintering aids whereby the choice of properties can be intentionally controlled for an intended use.